The present invention relates to die casting and more particularly to preheating components of the die when casting in order to improve the process and life of the die materials.
Die-cast motor rotors are universally produced in aluminum by pressure die-casting. That is a well-established and economical method. While copper possesses more attractive conductivity properties, leading to significantly greater motor efficiency, only small numbers of very large motors utilize copper in the rotors by mechanical fabrication. Tool steel molds that are used for the aluminum die-casting process have proved to be entirely inadequate when casting higher melting point metals including copper because they lack long die-casting life-in-service as they are unable to withstand the associated thermal stresses. Lack of a durable and cost-effective mold material has been the technical barrier preventing manufacture of the die-cast copper rotor, as well as die-casting for other high melting point and high heat capacity metals, such as stainless steel.
Die-casting, when it can be performed, is widely recognized as a low cost manufacturing process. For these reasons, die-casting has become the fabrication method of choice and aluminum the conductor of choice in almost all but the largest frame motors.
Die-cast copper rotors can provide advantages to motor manufacture and/or performance in at least the following ways: (i) improving motor energy efficiency in operation, (ii) reducing overall manufacturing cost, and (iii) reducing motor weight. Motor efficiency is a measure of the amount of power generated versus the amount of power input. Motors losses result from at least primary (i.e. stator winding) I2R loss (usually 34% to 39%), secondary (i.e. rotor) I2R loss (usually 16% to 29%), iron (core), friction and windage, and stray load. Since the late 1970s, when many aluminum stator windings were replaced by copper, there has been a focus on improving the operating efficiency of motors. Newer motor designs have recently improved efficiencies by increasing the amount of copper in windings, additional core and copper coil size, reduced windage losses, improved core steel, etc. However, the rotor remains die-cast aluminum because aluminum has a relatively low melting point of 660xc2x0 C. (compared to 1081xc2x0 C. for copper), and therefore that does not pose a hazard to the integrity of the mold, and because long-lived molds able to withstand repeated high thermal stresses are either not available or not commercialized.
Copper Conductive Rotors (xe2x80x9cCCR""sxe2x80x9d) reduce rotor loss, in addition to achieving overall motor re-optimization of iron, strays, etc. CCR-based designs show overall loss reduction from 15% to 20%. Aside from the higher efficiency (92.5% versus 91% for xe2x80x9cpremiumxe2x80x9d efficiency motors), CCRs in today""s premium energy efficient (EE) motors can cost approximately 5 to 8% less to manufacture than the current comparable EE motor, and/or produce a motor of comparable EE that weighs 5 to 10% less than the same energy efficient motor with traditional aluminum core rotors.
According to the study xe2x80x9cClassification and Evaluation of Electric Motors and Pumpsxe2x80x9d DOE/CS-0147 published February 1980, sponsored by the US Department of Energy, motors above ⅙ horsepower (xe2x80x9cHpxe2x80x9d) used about 60% of the electricity generated in the United States. When extrapolated worldwide, the potential economic and environmental benefits of improving the efficiency of motors by using copper rotors in place of aluminum rotors are substantial. Medium horsepower motors, that is, those in the one to one hundred twenty five Hp range (approximately 0.75 to 100 kW range), use about 60% of the electricity supplied to all motors in the US. Because of the proliferation of electric motors in this horsepower range, the projected energy savings by using the copper rotor motor is a significant national consideration. Efficiency increases (a function of motor size) from improved electrical conductivity are projected to result in total US energy savings in the year 2010 of 20.2 E+12 Btu/yr at only 10% market penetration and 143 E+12 Btu/yr at a market penetration of 50 to 70%.
CCR could be utilized to reduce rotor I2R losses in an existing motor design, replacing the existing die-cast aluminum rotor, without re-designing the motor to include more/better quality core, more stator windings, etc., which are the existing methods of improving motor energy efficiency in operation. CCR""s can be used in specific motors to achieve a multiplicity of intermediate combinations of these design advantages. For example, where a smaller efficiency increase is required, the CCR could be used to achieve some reduction in manufacturing cost (stator winding, core, etc.) than would otherwise have been the case with traditional aluminum die-cast rotor technology.
The problem encountered in attempting to die-cast copper motor rotors is thermal shock and thermal fatigue of mold materials. Thermal cycling of the mold surface limits the mold life even in aluminum die-casting. Even when the steel mold material is pre-heated, often by circulating hot oil through the die so that it reaches about 250xc2x0 C., the xcex94T with 1081xc2x0 C. copper still far exceeds the yield strength of steel. That results in cracking (xe2x80x9cheat checkingxe2x80x9d) of the die material. Over repeated casting shots, the tiny cracks grow into larger fractures.
Die-cast motor rotors are typically produced using aluminum because rotor fabrication by pressure die-casting in aluminum has proven to provide cost effective methods and materials for commercially feasible production runs. Copper rotors have typically not been produced because the melting temperature for copper is substantially higher than aluminum, resulting in thermal shock when the copper contacts the steel mold that greatly exceeds the yield strength of the mold material, which leads to commercially unacceptable rates of heat checking, i.e., short steel mold life-in-service, or cracking of the mold due to thermal stress. A low initial temperature of the die results in a large xcex94T at the surface of the die, and thus the stress in the die, on each shot. The high melting temperature, high heat of fusion, substantial latent heat and high thermal conductivity of copper combine to maximize the thermal shock.
Lack of durable and cost-effective mold material, and in particular a proven method for die casting with higher melting point electrically efficient materials has been a barrier preventing manufacture of the die-cast copper rotor. It is well known, however, that incorporation of copper for the rotor conductor bars and end rings in the induction motor in place of aluminum would result in attractive improvements in motor energy efficiency due to copper""s exceptional electrical conductivity.
It is therefore an object of this invention to provide a commercially feasible method for die-casting high temperature, high performance materials such as copper that will avoid the heat checking and cracking in the mold due to thermal shock of the mold that occurs under traditionally practiced methods of die casting.
It is a further object of this invention to provide processing conditions designed to withstand the copper motor rotor die-casting environment for an commercially and economically acceptable mold material life-in-service.
It is a further object of this invention to provide a die-casting apparatus to facilitate commercial die casting of copper motor rotors.
It is a further object of this invention to employ the die-casting apparatus to die-cast copper on suitable mold materials, such that the thermal shock to the mold material does not result in exceeding its yield strength.
As a starting point, the solution to the thermal shock problem lies in the use and creation of high temperature mold materials having thermal and thermo-elastic properties conducive to minimizing thermally induced strain. Even the most resilient known mold materials, however, cannot withstand repeated thermal shock associated with die-casting of molten copper at 1081xc2x0 Celsius. The present invention is directed to overcoming the current obstacles to die casting copper, by providing an apparatus and method capable of pre-heating the mold inserts to an elevated temperature to reduce the thermal differential between the die mold and the molten copper capable of overcoming the thermal shock problem and enable commercially feasible die casting of copper.
In particular, the present invention is directed to providing mold inserts including resistive heaters capable of pre-heating the mold to an elevated temperature to reduce the thermal differential between the die and the molten copper, then pre-heating the material to be cast to a temperature at which the thermal shock from the molten copper is less than the yield strength of the mold material into which it is being cast, injecting the material into the die-cast mold, and allowing the material to cool to a temperature below the melting point. As detailed below, the mold inserts that are the focus of the present invention are inside the master mold assembly, which assembly is generally made of steel and can weigh in excess of ten tons. The mass of the master mold assembly ensures that there will be adequate clamping pressure on the mold inserts to overcome the injection force of the molten material in operation and keep the mold inserts closed. The mold inserts, which are shaped and actually comes into contact with and forms the molten material, are made of a suitably strong material to withstand the heat of molten copper, for example. They are the portions of the master mold assembly that are pre-heated in order to prevent heat-checking, while the master mold, which is generally made of steel and absorbs the heat from the mold insert that it envelops, may be cooled in order to prevent the steel from re-annealing and gradually losing its shape. One or more mold inserts may be used, preferably two halves comprising the runner through which the molten material flows into the die, two inserts for the end rings, and a cylinder surrounding the core stack of laminations.
Preheating the mold inserts decreases the thermal differential between the molten copper and the mold material, lessening the thermal shock to the mold when contacted by the high temperature of molten copper. It is essential that the mold be pre-heated to a temperature at least at which thermal shock caused by the thermal differential between the molten copper and the pre-heated mold is below the yield strength of the material, otherwise cracking will occur. Ideally, the pre-heat temperature will be higher than that temperature to allow for anticipated flaws in the mold material or in the mold, which can lead to part of the interior of the mold inserts having a lower yield strength than the others. So long as the yield strength of the mold material is not exceeded as a result of the thermal differential when the molten copper is injected, there is no discernible benefit to pre-heating to any higher temperature to approach the melting point of copper, for example. Indeed, the higher the temperature to which the mold is pre-heated, the longer it will take for the molten copper to cool, extending the time for each casting, which is commercially undesirable. To the extent possible, then, the ideal temperature for pre-heating the mold would be just above that at which the thermal shock associated with the application of molten copper exceeds the lowest yield strength of the mold material. That would optimize the time period for cooling while still ensuring that the mold will not crack as a result of the thermal shock from molten copper.
This longer mold life removes the obstacle to mass production of copper motor rotors, and allows for commercially feasible production of rotors having die-cast copper conductor bars and end rings in the induction motor in place of the traditional aluminum, which yields attractive improvements in motor energy efficiency due to copper""s high electrical conductivity.